1. Field of the Invention
This invention relates to a resilient returning device for a power operated piston tool, more particularly to a resilient returning device having an elastic bellow-shaped body formed with a plurality of loop-shaped grooves.
2. Description of the Related Art
Referring to FIGS. 1 and 2, U.S. Pat. No. 6,824,035 discloses a power operated piston tool that includes an outer barrel 92, a guiding barrel 95 fitted in the outer barrel 92, a fastener guide 96 coupled to the outer barrel 92, a piston 91 mounted movably in the guiding barrel 95, a resilient returning device 93 disposed in the guiding barrel 95 around a piston shank 916 of the piston 91 and abutting against the fastener guide 96 and a piston head 915 of the piston 91, and a power-driving device 97 disposed at a rear end of the guiding barrel 95 and including a firing pin 971 and firing cartridges 972. After firing the firing cartridges 972 upon striking the striking pin 971, the piston 91 is driven by firing gases to drive a fastener 94 loaded in the fastener guide 96 into a wall and the resilient returning device 93 is compressed (see FIG. 2) for accumulating a restoring force to restore the piston 91 to its original position. Referring to FIG. 3, the resilient returning device is made from a rubber material and has a bellow-shaped body with a plurality of repeating arc-shaped pleats 931 that are serially interconnected. Each of the pleats 931 has two pleat halves 9312 that are moved toward each other during a compressing process of the resilient returning device 93 after firing as illustrated in FIGS. 4A to 4D. The two pleat halves 9312 cooperatively define an elbow 9311 therebetween. The resilient returning device 93 is disadvantageous in that a convex side (S) (indicated by a bold line in FIGS. 4A to 4D) of the elbow 9311 is considerably stretched when the resilient returning device 93 is disposed at a completely compressed position (see FIG. 4D). As a consequence, the convex side (S) of the elbow 9311 tends to break after the resilient returning device 93 is used for a period of time.